This invention relates to a passivated semiconductor pn junction which has a high electric strength, one area being heavily doped and being very thin, in particular for radiation detectors, and to a process for its production.
Passivated pn junctions for radiation detectors have already been described in the German Offenlegungsschrift No. 26 55 685.4 and the German Offenlegungsschrift No. 26 49 078.8-33 by the applicant.
Amongst the technological processes for the production of pn structures in Si, doping by diffusion and doping by ion implantation are of particular importance. In particular with the aid of the implantation technique, doping profiles of the most diverse structures can be obtained. The so-called flat pn junctions, which are required particularly in the manufacture of optical detectors and nuclear radiation detectors, are of great importance.
These pn junctions in most cases have a large surface area and, as a rule, have strongly asymmetric doping which is as abrupt as possible. The spacecharge region, which serves for detecting the radiation, extends almost exclusively into the semiconductor with low doping. The heavily doped part of the pn junction, which is made as thin as possible (about 0.1 .mu.m) in order to avoid absorption losses, serves as the entrance window for the radiation. In the case of radiation of short range, such as, for example, corpuscular radiation, passivation of the entrance window by SiO.sub.2 or Si.sub.3 N.sub.4 is in general not permissible. Such a passivation extends merely over the edge zone and serves to effect a reduction in surface currents or a stabilization against influences of the surroundings.
When the known planar technique is used, these pn junctions are produced, after the photolithographic opening of windows in the passivated semiconductor, by diffusion or by ion implantation. The breakdown voltages of such pn junctions are influenced inter alia by the radius of curvature in the edge zone of the doped layers.
In the case of diffusion, in contrast to the implantation technique, extensive doping is achieved also underneath the protective passivation layer. In this way a greater radius of curvature is obtained, which allows higher operating voltages. In pn structures obtained by flat diffusion, however, as in the case of doping by ion implantation, the radius of curvature of the edge zone is extremely small, and this leads to high local field strengths and hence to low breakdown voltages.
An enhancement channel between the protective layer and the semiconductor located underneath (surface enhancement channel) has an additional negative influence on the breakdown voltages. As a rule, this state is always obtained when n-Si of (111) orientation is passivated with thermally generated SiO.sub.2.
If, however, a surface depletion channel is present, the latter leads to a lowering of the edge field strengths, that is to say to higher breakdown voltages.